In zinc electroextraction which is carried out in an acidic sulphate medium the presence of small quantities of metal impurities (Ge, Sb, Ni, Co, As, etc.) results in difficulties in the process of electrocrystallization of zinc: lowering of the faradic efficiency of the electrocrystallization, stimulation of the release of hydrogen and redissolving of the zinc deposit. Thus, for example, at Ni or Co concentrations higher than 5 mg/l the efficiency rapidly decreases after a stable induction period, the length of which depends on the concentration of the impurity. The elements Ge and Sb have a particularly detrimental effect on the efficiency, even in very low concentrations (approximately 0.1 ppm) and practically without any induction period. The lowering of efficiency caused by an impurity generally goes in hand with a depolarization of the zinc electrode, after an induction period in the case of nickel or cobalt, but virtually immediate in the case of germanium.
Work aimed at remedying these difficulties is based on the use of additives in the electrolyte. The following additives have been investigated in particular:
lead (E. J. Frazer, J. Electrochem. Soc., 135, 1988, p. 2465) PA1 gum arabic (M. Maja et al, Oberflache-Surface, 24, 1983, p. 234) PA1 glue (D. J. Mackinnon et al, J. Appl. Electrochem., 17, 1987, p. 1129) PA1 liquorice (T. J. O'Keefee et al, J. Appl. Electrochem., 16, 1986, p. 913) PA1 2-butyne-1,4-diol (M. Sider et al, J. Appl. Electrochem, 18, 1988, p. 54) PA1 a molybdate (M. M. Jaksic, Surf. Coat. Technol., 28, 1986, p. 113) PA1 tetrabutyl- or tetraethylammonium chloride (D. J. Mackinnon et al, J. Appl. Electrochem., 9, 1979, p. 603) PA1 a mixture of ethoxyacetylenic alcohol (HOCH.sub.2 C.dbd.CCH.sub.2 OCH.sub.2 CH.sub.2 OH), triethylbenzylammonium chloride and polyethylene glycol (Chr. Bozhkov et al, Proceedings of the 7th European Symposium on Corrosion Inhibitors, Ferrara, Suppl. No. 9, 1990, p. 1211).
The ethoxyacetylenic alcohol, which must be present in a high concentration, is not a commercial product. Moreover, it has the disadvantage of being consumed during the electrolysis.